Add mobile styles to Sessions Home view

[themightymo]

I noticed that the home Sessions view at https://sessions.minnestar.org/home is difficult to read on my phone (Pixel 3a). These styles add some very basic mobile readability to the existing view.

• I'm not sure if 600px is the right breakpoint, but it's a start!
• I haven't done extensive testing on different views, because I'm not sure how to do that efficiently (lack of skills on my part and would love to learn how!).
• Since the sidebar is no longer a sidebar in this new view, I removed the background color. It could still use a little bit of design differentiation (header?, divider bar?), since it's a different content type (FAQ), but I didn't add that differentiation at this time.

Mobile view BEFORE:

Mobile view AFTER:

[themightymo]

There seems to be an issue with this code: while responsive styles load properly, they do not load when I'm testing devices in Chrome Inspector. I don't see "zoom:1" in the markup (a common cause of this issue with Inspector), nor was I able to find anything else that might cause this to happen (though clearly I missed something since it's happening). Any ideas what might be causing the mobile styles not to load when using devices in Chrome Inspector?

[pcantrell]

Thanks for this PR, @themightymo! Finally getting it merged. Only took us 4 years! I added a few hacks to make the session detail page look less broken on mobile as well.